Viking needed a method to determine its orientation (where it was pointing in space) in order to change itself to the required orientation. The Sun and the star Canopus were the two fixed points that it used to do this. Its photoelectric sensors found these stars, and the information enabled the spacecraft to fix its attitude. On occasions one or the other point was occluded by Mars, and at these times an inertial guidance unit provided guidance information.
At the tip of each solar panel there was an "attitude control jet". This device released nitrogen gas, stored in two 30-cm bottles, resulting in a slight movement. The gas release was controlled by a special subsystem devoted to attitude control.
